A considerable number of crystalline zeolitic alumino-silicates, both naturally-occurring and synthetic, are known in the art. Having crystal structures formed by the corner-sharing of SiO.sub.2 and AlO.sub.2 tetrahedral units, some zeolite species are known in which the Si/Al.sub.2 ratio of the as-crystallized lattice is as low as 2.0 and others in which the Si/Al.sub.2 ratio is as high as several hundred. In accordance with the now generally-accepted theory expressed by Loewenstein's rule [Amer. Mineralog. (1954) 39, 92] AlO.sub.2 tetrahedra are joined only to SiO.sub.2 tetrahedra, and hence the Si/Al.sub.2 ratios of zeolite frameworks cannot be less than 2.0. There is no theoretical restriction upon the upper limit of the Si/Al.sub.2 ratio.
While certain zeolite species, for example ZSM-5, can be directly crystallized hydrothermally to have a very wide range of Si/Al.sub.2 ratios, i.e., 5 to &gt;200, others such as the synthetic faujasites represented by the zeolite X-zeolite Y continuum have been limited to the relatively small range of 2.0 to about 6 for the as-crystallized compositions. A particular cesium-containing zeolite, CSZ-3, said to have the faujasite type of crystal structure, has been reported in which the as-synthesized form has a Si/Al.sub.2 ratio of as high as 7.0. Heretofore, attempts to change the as-crystallized Si/Al.sub.2 ratios, particularly in the case of the low-silica species, have been toward increasing the Si/Al.sub.2 ratios. The earlier efforts involved hydrolysis and extraction of framework aluminum to yield a more siliceous but defect-containing crystal lattice. More recently, procedures generally referred to as secondary synthesis have employed silicon tetrahalides or fluorosilicate salts to extract framework aluminum atoms and substitute in their stead silicon atoms. The primary goals of these investigations were to improve the hydrothermal stability of the crystal lattice and decrease the number of acidic sites associated with the AlO.sub.2 tetrahedra in order to render the catalytic activity more selective, particularly for hydrocarbon conversion reactions.
It is not always the case, however, that the more siliceous forms of the faujasite crystal structure provide superior performance. In certain common adsorption-separation processes, such as air separation, the basis for selective adsorption of one or more of the components of the mixture to be separated is the interaction of the local electrostatic fields in the zeolite with molecules of the mixture possessing permanent dipole or quadrupole moments. These interactions are extraordinarily complex and require, inter alia, that the particular combination of cation species making up the cation population of the zeolite be optimized in view of the various components of the mixture to be separated and the process conditions imposed. It is found that maximizing the number of cation sites in the zeolite adsorbent is often advantageous and accordingly it is necessary in such cases to maximize the number of AlO.sub.2 .sup.- tetrahedral units in the zeolite.
Zeolite X, having large pores interconnecting large internal cavities, and thus having the potential for adsorbing large amounts of selectively adsorbed molecules, is widely used in adsorption processes in which crystal degradation due to acid attack or hydrothermal abuse is not a significant problem. In addition the crystal structure of zeolite X permits its formation by hydrothermal crystallization in a form in which the number of AlO.sub.2 tetrahedra are essentially the same as the number of Si.sub.2 tetrahedra, i.e., the framework Si/Al.sub.2 ratio is about 2.0. In the synthesis of zeolite X, and also of the more siliceous zeolite Y, a variety of reagents can supply the silicon and aluminum incorporated into the crystal lattice as the tetrahedral oxide units. Suitable silicon-containing reagents include silica gel, silica acid, aqueous silica sols, amorphous solid silica and sodium silicate. Suitable aluminum-containing reagents include activated alumina, gamma alumina, aluminum trihydrate and sodium aluminate. The reagents within each group are not, however, exact equivalents, i.e., a zeolite product having a given Si/Al .sub.2 ratio and degree of purity cannot necessarily be prepared under identical reaction conditions and reagent proportions with each of the aforementioned sources of silica and alumina. There are, moreover, significant differences in the costs of raw materials so that some reagents are much preferred over others for the commercial manufacture of zeolites. In the case of zeolite X it has also been found that the reaction mixtures which can result in products having Si/Al.sub.2 ratios below about 2.5 are also capable of forming zeolite A as an impurity phase. To inhibit nucleation of the zeolite A structure, it is commonly the practice to include substantial amounts of potassium ions in addition to the usual sodium ions. The use of potassium ions has the disadvantage of adding to the cost of the synthesis mixture. It also results in a significant portion of the cation sites of the zeolite X product being occupied by potassium cations. In some instances the presence of these potassium cations is undesirable for the intended adsorption process, necessitating their removal by a post-synthesis ionexchange.